Fatigue of a structure may occur during repeated or cyclical application of loads on the structure. For example, the fuselage, wings, and landing gear of an aircraft are subjected to cyclic loads of constantly varying magnitude during takeoff, cruise flight, and landing. During takeoff, the wings of the aircraft deflect upwardly resulting in a bending load on the wings as the weight of the aircraft is transferred from the landing gear to the wings. During cruise flight, the fuselage is subjected to bending loads due to wind gusts and during maneuvering of the aircraft. Each maneuver and each wind gust may cause the fuselage to deflect or bend slightly resulting in a momentary increase in the tension stress, compression stress, or shear stress in the fuselage skin and related structure.
During descent from altitude, the wings are typically subjected to relatively high-frequency bending load cycles as the aircraft encounters low-altitude turbulence. Each time the aircraft lands, contact of the landing gear with the runway results in the transmission of relatively large loads to the fuselage as the weight of the aircraft is transferred from the wings to the landing gear. In addition, during each takeoff and landing cycle, the passenger cabin is pressurized and depressurized which cyclically loads the fuselage skin in tension.
To compensate for fatigue, structures are typically designed using previously-developed design rules in combination with stress-cycle (i.e., S-N) curves for a given material. An S-N curve of a material plots the quantity (N) of loading cycles of stress (S) of a given magnitude that will result in fatigue failure of the material. Unfortunately, S-N curves for a given material only represent the fatigue limits of a material subjected to stress of a constant magnitude. However, the magnitudes of stress that a structure is subjected to during service may vary greatly between loading cycles which may result in the structure having an actual fatigue life that may be longer or shorter than the predicted fatigue life.
For aircraft structures, the magnitude of the loading cycles in the predicted fatigue life may be more severe than predicted resulting in under-designed aircraft having a shortened fatigue life. Conversely, the magnitude of the loading cycles may be less severe than predicted which may result in over-designed aircraft that are heavier than necessary. The increased weight of the aircraft may result in reduced aircraft performance. In addition, the increased weight of the aircraft may reduce fuel economy.
As can be seen, there exists a need in the art for a system and method for accurately determining the fatigue life of an aircraft based on actual loading cycles.